Process for production of fertilizers

ABSTRACT

A process for the production of fertilizer by reacting sulfur oxide containing gas and water in a multiple state reactor to form sulfur containing acid and reacting the sulfur containing acid with a basic salt which may be solubilized as a result of the acid-base reaction, the basic salt selected from the group consisting of calcium, ammonium and potassium ions, the calcium being derived from a salt selected from the group consisting of phosphatic and nitrogeneous salts, to form a fertilizer selected from the group consisting of phosphatic, nitrogeneous and potassium fertilizers and combinations thereof. The combined sulfur oxide absorption and solubilization of basic salts is conducted at the solution pH of 2.5 to 5. To enhance the further absorption of the sulfur oxide, a solution pH of 3 to 8 is used. This process is suitable for the utilization of sulfur oxides resulting from the combustion of high sulfur containing fossil fuels and sulfur producing chemical processes. The process of this invention enables the utilization of low concentration sulfur acid in the solubilization of phosphate rock utilizing a multiple state reactor. To obtain solubilization of untreated phosphate rock, oxidation of the sulfur dioxide to sulfur trioxide and conversion to sulfuric acid is preferred while solubilization of pretreated, defluorinated phosphate rock may be achieved by sulfurous acid formed from sulfur dioxide thereby eliminating the need for oxidation. Lower grades of phosphate rock are suitable for use in the process of this invention.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our copending applicationSer. No. 677,778, filed Apr. 16, 1976, now U.S. Pat. No. 4,073,634.

Phosphate fertilizers have become very important in the agriculturaleconomy throughout the world. A major phosphate source for suchfertilizers is natural phosphate rock. However, in its natural form suchphosphate is nearly insoluble in water. To utilize the phosphate fromnatural phosphate rock sources, various acidulation processes utilizingsulfuric, phosphoric or nitric acids have been used.

While phosphoric acid can be produced by the action of eitherhydrochloric acid or nitric acid on the natural phosphate, suchprocesses have not been satisfactory since the soluble nature of thesalts in the resulting products make separation of phosphoric acidcommercially impractical. When natural phosphate rock is treated withsulfuric acid, the products are phosphoric acid and gypsum (hydrouscalcium sulfate) and other sulfate salts. Due to their insolubility, thesulfate salts may be readily separated from the phosphoric acid. Priormethods of fertilizer production have employed sulfuric acid in themanufacture of phosphoric acid by the well known "wet process."

Super phosphate is the term generally applied to the product obtained bytreating finely ground phosphate rock with sulfuric acid and has about16 to 20 percent available P₂ O₅. In the manufacture of super phosphate,a source of concentrated sulfuric acid must be available as well assources of high grade rock phosphate. The reaction is dependent upongood liquid-solid reactant mixing. The lower grades of phosphate rockwhich contain higher percentages of impurities are not satisfactory forprior processes for the manufacture of phosphoric acid or otherphosphatic fertilizers. Many of the impurities contained in the naturalphosphate rock react with sulfuric acid and the consumption of sulfuricacid becomes too great. Further, due to the poor reaction kineticsinvolved in the two phase liquid-solid reaction system in themanufacture of phosphoric acid a curing or tempering period requiring asmuch as several days is necessary. In the manufacture of non-granularphosphatic fertilizers a long curing process, as much as several months,is required to complete the reaction reducing the free sulfuric acid toa minimum. In the production of granulated phosphatic fertilizers usingphosphatic rock the amount of good liquid solid reactant mixing isimportant to minimize bag rot or other corrosive action from excess freeacid. The hardness of fertilizer granule is dependent upon the avoidanceof unreacted sulfuric acid.

Triple super phosphate providing a highly concentrated phosphaticfertilizer containing from 44 to 46 percent P₂ O₅ has been manufacturedby the decomposition of phosphate rock using phosphoric acid. In thiscase the citrate soluble phosphate is obtained using only expensivephosphoric acid.

It is an object of this invention to overcome many of the disadvantagesof prior processes for production of phosphatic fertilizers.

It is another object of this invention to provide a process for themanufacture of fertilizer from relatively low grade natural phosphaterock sources.

It is another object of this invention to provide a process forproduction of fertilizer which utilizes the calcium present in naturalrock phosphate sources.

It is still another object of this invention to provide a process forthe production of fertilizers which may be either solid, liquid orsuspension.

It is an object of this invention to provide for a more rapid reactionthan previously obtained between untreated phosphate rock and sulfuricacid in a heterogeneous gas-liquid-solid mixture.

It is an object of this invention to provide for reaction betweenpretreated defluorinated phosphate rock and sulfurous acid.

It is another object of this invention to provide a process for theproduction of fertilizer by utilization of dilute sulfur containing acidproduced from sulfur oxides obtained from the burning of sulfurcontaining natural fuels.

It is another object of this invention to provide a process for theproduction of fertilizer by utilization of sulfur containing acidobtained as a by-product of sulfur generating processes such as woodpulp digestion tanks.

It is an object of this invention to provide a process for production offertilizers at an inexpensive source of sulfur oxides which is also ingeographical proximity to the user of the fertilizer.

It is another object of this invention to produce phosphatic or otherfundamental plant nutrient chemicals which are readily ammoniable.

It is still another object of this invention to render the ammoniated orunammoniated phosphate or other basic plant nutrient readilygranulatable.

It is yet another object of this invention to make use of the sulfuroxides from burning of sulfur containing natural fuels using phosphaterock in conjunction with anhydrous ammonia without loss of free ammoniafrom the reaction system.

It is another object of this invention to provide a process for themanufacture of phosphatic fertilizer wherein sulfur dioxide may beconverted to sulfurous acid which effectively solubilizes pretreateddefluorinated phosphate rock.

These and other objects will become readily apparent by a reading of thedetailed disclosure and reference to the figures showing preferredembodiments wherein:

FIG. 1 shows schematically one embodiment of a process for theproduction of phosphate fertilizer according to this invention utilizinggroup phosphate rock;

FIG. 2 shows schematically one embodiment of a process for theproduction of phosphate fertilizer using unground rock phosphateaccording to this invention using product recycle in conjunction withthe granulation to form the solid finished product; and

FIG. 3 shows schematically one embodiment of a process for theproduction of phosphate fertilizer according to this invention usingfluid product recycle.

This invention utilizes the sulfur oxides obtained from the burning ofsulfur containing fossil fuels such as coal or oil in a conventionalboiler or sulfur oxides obtained as stack gas effluents from anyindustrial plants such as the chemical, paper, refinery, fertilizer,steel or coke operations provide. While the preferred embodiments willbe described with respect to sulfur oxides obtained from the burning ofcoal, it will be understood that sulfur oxides may be provided by anyother suitable source. It is readily apparent that the process of thisinvention enables the use of high sulfur content coal, sulfur content ofhigher than about two percent being preferred. Presently there are greatquantities of coal which are not being utilized due to its excessivesulfur content. There is increasing demand for energy sources other thandesulfurized oil or gas. The satisfactory utilization of high sulfurcontent coal and oil would alleviate such demands most effectively andpermit use of existing combustion equipment with slight modification.Very expensive techniques for the removal of pollutant sulfur oxidesfrom such stack gases have not proved satisfactory. This inventionprovides a process which encourages the burning of high sulfurcontaining fuels which in many instances, is considerably moreeconomical to the boiler operator due to both higher heat and lowertransportation costs. This invention also provides a process for theburning of high sulfur fuels with the utilization of the sulfur oxidesthereby produced and resulting in safe levels of final stack effluentwith respect to the sulfur oxides content permissible by the appropriateregulations, in fact, usually lower than the sulfur oxides permitted. Itis, therefore, economically feasible for power generating companies orother industries dependent upon sulfur containing coal and oil boilerutilization to utilize the process of this invention for the productionof fertilizer as a by-product of the burning of high sulfur coal or oiland to provide local agricultural growers with grades of fertilizermaterials well suited to good farming practice thereby reducing the highcost of transportation now prevalent in the fertilizer industry. Someindustries are being forced, to effect oil and gas fuel conservation, toconvert to coal utilization. Frequently, utilization of the process ofthis invention permits use of local fuel again reducing hightransporation costs. The investment in equipment for use in thisinvention provides both a means for SO_(x) removal from stack effluentsand fertilizer production. This is very important since the fertilizerindustry is continuously attempting to balance the facilities for supplyof fertilizer to satisfy the demand.

The stack gas containing sulfur oxides and fly ash from a conventionalcoal fired boiler at approximately 380° F. to 450° F. may be passedthrough conventional equipment for fly ash and solids separation fromthe gas stream. The fly ash separator may be a single stage of theheterogeneous reactor more fully described later in this specification.In instances where it is practical to mix the phosphate rock with thecoal prior to being fired to the boiler, it would not be desirable touse the fly ash separator.

In one embodiment of the process of this invention, which will befurther explained in greater detail, the SO₂ from the stack effluent maybe converted to sulfurous acid and used to solubilize pretreateddefluorinated phosphate rock. In another embodiment of the process ofthis invention in which untreated phosphate rock is used, it isdesirable to oxidize SO₂ from the input gas stream and hydrate so thatthe sulfate ion is formed and there is minimal SO₂ in equilibrium withthe solution. Maximizing oxidation of SO₂ decreases the probabilitiesfor SO₂ escape to the atmosphere. H₂ SO₃ acid solutions or saltsolutions which have the SO₃ ⁼ radical tend to decompose forming SO₂.Therefore, it is desirable to oxidize the sulfur oxides as completely aspossible to form the sulfates which do not decompose. This oxidation canbe effected by various methods, such as by use of an oxidation catalystin direct contact with the stack gas, especially, if the solids had beenremoved; or the oxidation can be effected in a separate air-solutionabsorption tower using countercurrent air; or in a heterogeneous reactorcocurrent absorber such as referred to in other portions of thisapplication; or in any absorption device which brings air into contactwith the liqueur resulting from the absorption of the sulfur oxidegases; or oxidizing salts may be added to the above solutions to enhanceformation of sulfuric rather than sulfurous acid or salts.

The hot gas is passed into a direct contact cooler-absorber reactorwhere some of the sulfur oxides are absorbed by direct contact by thegas to a cooling liqueur which can be water or water to which a chemicalhas been added to increase the pH to promote greater absorptionreaction. The solution which remains after water evaporation as a resultof cooling the gas is fed to a settling tank from which the decantliquid can be recycled to the cooler-absorber reactor. The directcontact between the sulfur oxides containing gas stream and the liquidcools the gas stream as a result of the evaporation of some of theliquid phase. The evaporative cooling is conducted to about the point ofsaturation of the gas with the vapor and then the gas stream passed to areaction-absorption tower either in cocurrent or countercurrent flowrelative to an absorbing liqueur.

The function of the reaction-absorption tower is to bring into solutionwhatever sulfur oxides, and possibly nitrogen oxides, were not reactedin the contact cooler-absorber. In this reaction-absorber chemicalsolutions with higher pH are used. The gas having been stripped of theacid anhydrides then exits through a demister and a blower out a stackto the atmosphere. In some installations, a reheat to elevate thetemperature of the effluent gases above the saturation level may bespecified prior to exhausting.

One chemical utilized to accomplish the sulfur oxides removal is thecalcium found in phosphate rocks. The calcium in its original phosphaterock form is not water soluble and therefore, must be rendered solubleby reacting it with acid. In this process the sulfuric and sulfurousacids formed from the sulfur oxides contained in the flue gas, coupledwith phosphoric or various phosphatic acids or nitric acid or variousnitrogeneous acids to increase the reactivity of the acid solutionrelative to the rock, may be used to digest the phosphate rock andsolubilize the calcium ion.

The chemical reaction between the acid and ground phosphate rock may beconducted in a suitable heterogeneous reactor. The rock phosphate may beused in untreated and unground form and the reaction between the acidand phosphate rock conducted in a ball mill. The function of the ballmill is to promote reaction by mechanically fracturing any coating ofthe phosphate by calcium salts. When a mixture of the phosphatic acidsor nitrogeneous acids is used in combination with the sulfuric andsulfurous acids it may be desirable to use the mechanical grinding whenunground phosphate rock is introduced to the process. Another means ofreacting the unground phosphate rock with the acids, if the boileroperation is not impeded or erosion does not prohibit, such as withunderfeed or traveling grate stroker systems, is to add untreatedphosphate rock to the coal fired boiler itself and to grind the rock andthe klinker in the ball mill reactor; or the rock-klinker mixture can bedry milled prior to addition to the towers. The acidulation can also beconnected by contacting pre-ground phosphate rock with the mixture ofacids. The powdered or ground phosphate rock may be introduced intoswirling phosphoric acid in a cone for reaction prior to theintroduction to reaction tanks. If granulation is not desired, theslurry from the acidulation cone is passed to a moving slat or beltconveyor for hardening and cutting to form the pulverized or Run-Of-Pile(ROP) product. Another alternative is to bring the acidulated rock intoa granulation system. If the oxidation in the towers does not result instrong enough sulfuric-sulfurous acids, then concentrated phosphoricacid must be combined with those acids to increase the solubilizationrate of untreated phosphate rock. If desired, nitric acid may also beused. The same consideration with respect to solubilization holds trueregardless of the manner in which the slurry is formed. Another means ofsolubilization is adding ground, untreated phosphate rock with excessair prior to the cooling tower. Ground phosphate rock may also be addedto the top of the heterogeneous reactor. The rock should be ground to atleast the commercial grind of 90 percent minus 100 mesh.

Phosphate rock suitable for use in this invention, whether untreated orpretreated by defluorination, has B.P.L. down to as low as about 50.B.P.L.'s about 55 to 65 are particularly suitable for use in thisinvention. B.P.L.'s of 66 and less are considered low grade phosphaterocks which are not suitable for presently used processes for productionof fertilizer but are suitable for the process of this invention. Thelow cost of the low grade phosphate rock makes the process of thisinvention very economically attractive. Rock having B.P.L.'s higher than66 are also suitable for the process of this invention.

When the calcium from the phosphate rock is made available by acidattack for neutralization of the sulfur oxides by any of the previousmeans, the entire calcium requirement for the process may be supplied bysuch means instead of the necessity of providing costly calcium fromoutside sources. This effects savings by not having to purchase thecalcium from an outside source. If additional basic ions are needed toremove the acidic ions from the hydration of the oxides of sulfur,ammonia may be added to the system or any other basic low cost plantfood nutrient may be used. As an example, if the economics permit,potassium carbonate is another basic or buffering ion.

The slurry resulting by the solubilization of the rock phosphate is thenmade up of calcium, sulfate, sulfite, phosphate and possibly nitrateions, other ions brought in with the desired acids, plus ions releasedfrom the rock as a result of the solubilization. If nitrogen is desiredin the end fertilizer product, the cheapest source is anhydrous ammoniawhich can be fixed by the acid slurry. The anhydrous ammonia may beadded to the slurry to neutralize it to the point where the acidity willnot create handling or storage problems. For example, granules havingtoo high a free acid content tend to be soft even after drying. Ifbagged, the excess acid attacks the bag causing what is referred to as"bag rot." If granules with excess acid are handled in bulk and stored,solutions are formed on their surface upon drying, result in thegranules knitting causing caking. The favorable economics in thisneutralization step result from the fact that fixed nitrogen or thatexisting in solid form, has a higher sales value than the originalgaseous ammonia. Also, the production of diammonium phosphate providesan excellent buffer using pretreated, defluorinated phosphate rock,providing an excellent solution for maintaining a low SO₂ vapor pressurein equilibrium with it.

The ash that was retained in the slurry contains minerals whichoriginally were derived from plants later petrified to form the coal.These chemicals have a two fold value. They are also slightly basic aswas evidenced by the manufacture of soap in the past by leaching thesechemicals from the ashes and saponyfying fats to make soap. However, themost significant contribution is that these minerals that contributed tothe strength and health of these prehistoric plants are capable of doingthe same for present day crops. These are normally known in the industryas trace nutrients, or micro-nutrients. This also gives added value tothe fertilizer product. The most significant contribution, economy-wise,is that this permits the operator of the boiler to dispose, in aneconomic fashion, of the fly ash by having it returned to the soil whilecontained in the fertilizer. Otherwise, the operator would have to spendmoney disposing of the ash in addition to the cost of abating thepollution.

This invention uses calcium derived from phospate rock to react withacids of sulfur which are very economically available. It has previouslybeen considered impractical to solubilize phosphate rock with the verydilute sulfuric and sulfurous acids that prior scrubbing processes madeavailable. The use of strong phosphoric acid by adding concentratedphosphoric acid to the weak acids of the sulfur oxides, the mixture ofwhich increases the temperature and increases the capability to attackthe rock making the calcium ion available, has been found advantageous.Use of weak sulfur acids derived from stack gas effluents to cut, ordilute, strong acids permits greater generation of fertilizer phosphatesthan the use of the strong acid alone.

In prior practices, concentrated sulfuric acid was added to phosphaterock to make single super phosphate. The resulting acidulate analyzedapproximately 20 percent P₂ O₅ when cured. Concentrated phosphoric acidmixed with the rock in proper stoichiometric proporations resulted in 46percent P₂ O₅ analysis when cured.

A third commercial method, one process of which is referred to as thePrayon Process, results in the manufacture of phosphoric acid usingproper stoichiometric amounts of sulfuric acid and phosphate rock.Enough sulfuric acid is used to precipitate all of the calcium in thephosphate rock as sulfate and other insoluble salts originating from therock. The slurry is then conveyed to a moving pan filter, and theliqueur is separated from the solids which is called black phosphoricacid, as opposed to furnace grade which is made from the burning ofphosphorous and the resulting P₂ O₅ gas is hydrated in water to formwhite, or clear, phosphoric acid. In the manufacture of the black acid,the filtration is becoming more and more difficult because the grade ofrock available is deteriorating. The process of the present inventionavoids this costly separation.

The process of the present invention can use lower grade phosphate rockthan existing fertilizer manufacturing processes. The process can uselow grade rock because there is no need to separate sulfates (calcium)from the resulting acid, and because of the reduced requirement ofphosphoric acid or costly concentrated sulfuric acid input to theprocess. Exemplary of the process of this invention 25% of the P₂ O₅ inthe phosphate fertilizer may be derived from the phosphoric acid and 75%from the phosphate rock in the commercial version of our process. Theamount of phosphate rock to be used is that necessary to supply calciumto neutralize all of the sulfur oxides available from flue gases orother inexpensive sulfur oxide sources. Therefore, when the resultantslurry is reacted with ammonia, the benefit of the phosphate that wasstripped of its calcium ion is derived in tying up the ammonia. Thephosphoric acid used to assist in the acidulation is then available toreact with the ammonia in the same way as when diammonium phosphate isproduced. If adequate oxidation of the sulfur dioxide to the trioxideresults in the case of untreated phosphate rock enabling thoroughhydration to result in sulfates production and a small amount ofdilution water can be used or enough recycle permitting the acidconcentration to rise above 40%, then very little phosphoric acid isrequired to be supplied to a ball mill reactor. In this case the ballmill is used as a reactor in connection with the solubilization to speedup the reaction to a matter of minutes by physically assisting thereaction through a combustion of mixing and grinding.

The process as schematically shown in FIG. 1 utilizes untreatedphosphate rock. FIG. 1 starts with a single stage dry particulateremoval apparatus to separate the fly ash. This is optional. Also, ifground phosphate rock fluidized with excess air were to be introducedinto the evaporative cooler, it would be brought into the cooling towerfollowing the dry separator as shown. In the cooling tower the sulfuroxides contained in the stack gas are absorbed in the droplets of thesolution emanating from the cooling sprays, and the acidified dropletsin turn react with the fine particles of phosphate rock. Thissolubilization of the rock strips the calcium from the phosphate rockleaving a phosphatic slurry and calcium salts of the various sulfuroxides. The cooling tower could be replaced by any effectiveheterogeneous reactor such as a single stage heterogeneous reactor asshown in FIG. 1 without an impingement plate. The slurry from thecooling tower goes into a settling tank.

In settling tank I, the solids are allowed to drop to the bottom and theclarified liquid is siphoned from the top and recirculates to thecooling tower so that whatever calcium ions are in solution can furtherreact with the sulfur oxides that have been hydrated to acids. If thedegree of oxidation of the sulfur is not sufficient to absorb more thanapproximately 25 percent of the sulfur oxides at the cooling tower stagethen strong commercially available phosphoric acid must be added asshown in figure to settling tank I to accelerate the acidulation of thephosphate rock.

The gas that had a part of its sulfur oxides removed and is nowsaturated with the vapor passes to the heterogeneous reactor.

These gases could also include particles of ground rock which had notbeen agglomerated or reacted within the cooling tower. Also, additionalground phosphate rock may be fluidized with air and added to thesegases. The mixture enters into the top of the heterogeneous reactor intowhich is supplied solution containing basic ions to take out theremaining sulfur oxides by reacting these oxides with the solution.Also, whatever particles of solid ground rock carried over, or added,would be agglomerated and brought down together with the solution intothe settling tank II. Here again, if the concentration of the sulfuracid would not permit a high enough percentage of rock solubilization,strong commercially available phosphoric acid would have to be added tosettling tank II. This invention permits starting with a dilute sulfurcontaining gas stream resulting in production of dilute sulfur acids,less than 60 weight percent sulfur acid, which has not previously beenused commercially to our knowledge. Again, the solids would be allowedto settle to the bottom and the clear decant would be recycled into thehetergeneous reactor.

The gas, now having been cleaned of the sulfur oxides, the fly ash andphosphate rock particles is pulled by the blower into a demister andblown out of the stack. A re-heater (not shown) might be requested bysome boiler plant operators to elevate the gas effluent temperatureabove the dew point.

The pH in the heterogeneous reactor is controlled by the dissolvedcalcium from the phosphate rock and may be further controlled byaddition of anhydrous ammonia, potassium carbonate or any othereconomical basic plant nutrient. A range of pH of from 3 to 8 may beused depending upon degree of sulfur oxidation, pH in settling tank I,supplemental acid used, and sulfur content of the coal. Further, toincrease the collection efficiency of the sulfur oxides, some of thesolution from settling tank II can be brought back to settling tank I,controlled by valve V₁, to increase the pH in settling tank I or viceversa. If the higher pH is held in settling tank II, a range of pH of2.5 to 5 could be maintained in tank I, again depending on degree ofsulfur oxidation, pH in settling tank II, supplemental acid use, andsulfur content of the coal. Under certain conditions of sulfur contentin the coal, tolerable SO₂ losses, degree of sulfur oxidation orphosphate rock pretreatment and defluorination, and supplemental acidused, the pH in both tanks may be maintained the same, for simplicity ofoperation and selection of structural materials. For the purpose ofcontrolling pH and reaction of the rock between settling tanks I and II,the tanks are interconnected in such a way that flow of solids orliquids can occur between them. When pretreated, defluorinated phosphaterock, as will be described in more detail, is used with oxidation of SO₂to SO₃, the higher pH solution from settling tank II may be added tosettling tank I to control the pH of the spray solution in the coolingtower. When pretreated, defluorinated phosphate rock is used to absorbthe sulfur oxides as they exist in the boiler exhaust, settling tank IIwould be split to provide an additional reactor tank for the purpose ofadding ammonia to control the pH of the heterogeneous reactor andcooling tower.

The bottoms are taken out of both settling tank I and II as solid-liquidslurry combined and brought into the pre-neutralizer or granulator orseparate between the two, dependent upon the percentage of solids in theliquid. The more liquid, the more would go into the pre-neutralizer.This is controlled by valve V₂. Here again, whatever additionalsolubilization of rock is desired, strong phosphoric and, if desirablefrom a formulation and vaporization viewpoint, strong nitic acid asshown in FIG. 1, can be added to the pre-neutralizer to promote theadditional solubilization and evaporation. Continued solubilizationwould occur in the granulator and could be completed in the drier, ifdesired. When additional anhydrous ammonia is added to thepre-neutralizer along with phosphoric acid the heat of reaction boilsoff the liquid. If there is a large amount of solids, as compared to theliquid, in the solid-liquid slurry from the settling tanks, it would bebrought directly into the granulator where reaction between anhydrousammonia and other acids takes place without the need for the extravaporization from the pre-neutralizer. If an N-P-K, or completegranulated fertilizer is desired, then potash is added to the granulatorin amounts required by the desired analysis. As per standard industrialfertilizer practice, the material flows from the granulator to a drierto a cooler, is sized and dependent upon the amount of recycle requiredon sized product might be added to the oversized, ground, mixed with"fines" or undersized and recycled to the granulator. Here again, ifthere is not much liquid then the amount of recycle is decreased and maynot require grinding of on sized product.

The process can be operated to produce a liquid fertilizer product baseas shown in FIG. 1 by adjusting valve V₃ to permit some of therecirculating liqueur to be taken out of the system. To this could beadded white potash, uncoated ammonium nitrate, or any other watersoluble plant nutrients in an agitated tank (not shown) for formulationto the grade desired as per standard commercial practice. Whenpretreated, defluorinated phosphate rock is used, the high reactivitypermits most of the stream to be withdrawn as liquid fertilizer, ifdesired. This would eliminate the pre-neutralizer, granulation plant andassociated equipment thereby reducing both capital and operating costs.

A suspension fertilizer base may be obtained by proper adjustment ofvalve V₆. Plant nutrients and agents required to stabilize thesuspension could be added in an agitated tank (not shown). Some of thevery fine silicious materials originating from the fly ash and thesolubilized rock add to this suspension stabilizing phenomena. As seenfrom FIG. 1, liquid suspension and solid fertilizers can be produced bythe process of this invention dependent upon the final product demand.

The heterogeneous reactor used in the process of this invention may beany suitable apparatus which promotes rapid chemical reaction ofreactants in heterogeneous gaseous-liquid-and/or solid state such as aturbulent bed packing absorber and other reactors designed to avoidplugging known to the art. One particularly suitable apparatus isdescribed more fully in U.S. patent application Ser. No. 677,750,Heterogeneous Reactor and Process, L. J. Pircon, filed Apr. 16, 1976,now U.S. Pat. No. 4,073,634.

The heterogeneous reactor is shown in FIG. 1 defined by outer casing 10.The cross-sectional shape of outer casing 10 is preferably cylindrical,but may be square, rectangular, triangular, hexagonal, or othersymmetrical polygon shape, but other geometrical shapes symmetrical withrespect to the axis of the apparatus are satisfactory, the principalrequirement being that it enclose the apparatus in generally liquid andgas type relationship while providing controlled gas flow through theinterior portion. To allow maximum flexibility in the utilization andmaintenance of the heterogeneous reactor casing 10 may be fabricated insections having sections having flanges as shown by 11 and 13 at eachend for rigid coupling to adjacent casing sections having like flanges12 and 14. Instead of the flanges as shown in FIG. 1, any suitablecoupling means may be utilized. To allow for maximum economy of originalfabrication and installation of larger units the sections may be weldedprior to shipment and erection. FIG. 1 shows a two stage heterogeneousreactor.

The heterogeneous reactor is arranged with its axis vertically havingthe reactant solid-liquid-gas inlet in the upper portion. The inlet maybe in either a vertical or horizontal position. The reactant flow issupplied to the top of casing 10 through the inlet at a velocity andpressure sufficient to carry it through the apparatus. The inletpressure is negative relative to the atmospheric exhaust in mostinstances so that the blower inducing the flow is not affected byabrasion due to any solids which might be contained in the inlet gas orchemical attack by corrosive components. The apparatus is a low pressureapparatus and generally casing velocities may be in the range of about200 to about 1200 feet per minute prior to introduction into nozzle 21.

Spray 41 may be located in the central portion of inlet to cylinder 10and introduces liquid or solid reactant, adsorbent, absorbent or liquidcoolant in droplet form to the reactant stream, the droplets beingpreferably in the order of about 40 to about 1500 microns in diameter.Larger droplets may be desired to compensate for evaporation whenevaporative conditions exist. Spray 41 is preferably a solid cone spraywhich by itself or in combination with several like it arranged in apattern permitting the introduction of droplets of water across theentire cross section of the pollutant gas stream prior to entry of thegas stream into cone 21. Different sized liquid droplets are desired toprovide maximun differential accelerations, decelerations and velocitiesthrough the apparatus, thus increasing reaction. It is desired that thespray pattern extend across the full area of entrance 25 of nozzle 21and any suitable pattern of sprays or multiple sprays is satisfactory.Spray 41 may also be used to introduce solid particles of the abovespecified sizes to the reactant stream at the entrance 25 of nozzle 21.

The reactant containing heterogeneous solid-liquid-gas stream entersconverging nozzle 21 through entry 25. It is preferred that the entry beround and the nozzle conical, but other geometrical shapes symmetricalwith respect to the axis of the apparatus are satisfactory. The coneratio, defined as the effective cross-sectional area of the entrydivided by the effective cross-sectional area of the outlet, should beabout 2 to about 64, about 2 to about 36 being preferred. By effectivecross-sectional area is meant the area at 90° to the axis of gas flow.

The length of the converging portion of the nozzle is determined by theangle of convergence shown as A in FIG. 1 and the nozzle ratio asdefined above. It is preferred that the mean angle of convergence beabout 6° to about 20°, about 10° to about 16° being preferred. By meansangle of convergence is meant the angle measured between a straight linedrawn from the entry to the outlet and a vertical line as shown by A inFIG. 1. The sides of nozzle 21 do not need to be straight, but may besomewhat convex or concave.

The distance from outlet 24 to the impingement surface 31 should beabout 1.3 to about 2.5 times the diameter of outlet 24, about 1.6 toabout 2.0 being preferred.

A suitable impingement plate is shown as 31 in FIG. 1. Impingement plate31 is of sufficient size to have substantially all of the particulatematter from nozzle exit 24 impinge upon it while affording sufficientarea between the impingement plate and cylinder 10 to allow passage ofthe gas around impingement plate without appreciable pressure drop.While impingement plate 31 is shown as a flat plate, a slightly concaveplate to facilitate the passage of gas around the edges and tofacilitate the removal of particulate matter may be utilized. Forreactions not requiring separation of solid and liquid phases from thegas phase, or mass transfer phenomena such as associated withevaporative processes such as take place in the cooling tower, animpingement surface would not be needed.

Additional sprays may be suitably located above impingement plate 31 sothat the spray therefrom washes particulate matter off impingement plate31 for progress through the appratus and discharge at the bottom. Suchsprays may be multiple sprays located around the periphery ofimpingement plate 31 or a satisfactory spray may be located in thecentral position. When sufficient fluid is used, the impingement surfacewill be the fluid itself and the particulate matter will not strike oradhere to the impingement plate, but will be entrapped in the fluid. Theessential criteria of the sprays upon impingement plate 31 is that theyprovide sufficient fluid with sufficient force and direction to keepimpingement plate 31 relatively free of particulate matter. The reactormay also be operated without the supplemental sprays to clean theimpingement surfaces.

Because of the unitized construction of the apparatus of this invention,as shown in FIG. 1, multiple nozzle-impingement means stages may bereadily placed one on top of the other, resulting in the series of threeunits as shown in FIG. 1. One to about 6 of the series connected stagesof nozzles are suitable for heterogeneous reactors for use in thisinvention. Preferably 2 to 4 stages are utilized in series. The numberof stages is controlled by the difficulty of reaction of the reactants,and with especially difficult materials, a greater number of stages maybe necessary. This could also be influenced by the angles of convergenceor effective cross-sectional area ratios of the nozzles.

Beneath the impingement plate of the bottom stage is reservoir 15 forremoval of the liquid and slurry. Exit means for the removal of theclean gas are also provided beneath or adjacent bottom-most impingementplate 33 and shown in FIG. 1 as conduit 16. Either within the apparatusor external to the apparatus it is preferred to have demister 17 in theclean gas effluent line to remove fine droplets of liquid remaining inthe gas stream together with any solids or gases trapped by suchdroplets.

The vertical arrangement of the converging nozzles is particularlyadvantageous since using such an apparatus with a demister having anozzle ratio of 4 and a nozzle angle of 12°, the pressure drop in onenozzle is 3.5 inches of water; with two nozzles in series is 5.7 inchesof water; with three nozzles in series is 7.0 inches of water; and withfour nozzles in series is 8.3 inches of water when an inlet valocity ofapproximately 2100 feet per minute was used. When the same apparatushaving two nozzles in series was used for removal of SO_(x) andfertilizer production according to this invention with an inlet velocityof approximately 400 feet per minute, a pressure drop of 0.8 inches ofwater was obtained with excellent process results. Thus, it is seen thatthe pressure drop of the vertical series of nozzles is advantageouslyless than cumulative.

The second stage, as shown in FIG. 1, is identical in configuration tothe first stage. It is recognized, however, that the water or liquidchemical supplied to both the nozzles preceeding the cone entrance andthe nozzles supplying liquid to the impingement surface of the samestage or of different stages may be individually controlled. That is,the volumes may be different and the liquid used may be different ineach instance.

The passing of the liquid, solid and gaseous reactant in the streamthrough nozzles such as 21, promotes intimate contact between theliquid, solid and gaseous reactant and results in desired high reactionrates. It is believed the high reaction efficiency of the heterogeneousreactor and process is due to differential velocities and differentialacceleration and deceleration achieved by the combination ofnon-compressible matter passing with the compressible gas through nozzle21 with the opportunity for relatively great expansion following exitfrom nozzle exit 24. In the reactant containing stream there is a sizerange of compressible and non-compressible matter. Additional particlesadded to the gas stream by addition of solids or liquid droplets areprincipally non-compressible as desired to increase the non-compressiblecomponent of the gas stream. Spray 41 may be used to introduce a wideselection of liquid or solid particle sizes to the gas stream andtogether with a relatively wide span of liquid or solid particle sizesin the inlet gas stream, promote extremely high collision rates and highcompressible gas rates flowing past the non-compressible particles anddroplets resulting in very highly efficient reactions.

In order to minimize the height of the heterogeneous reactor as shown inFIG. 1, multiple cones may be placed in each stage.

Another preferred embodiment of this invention is shown in FIG. 2providing for the use of untreated, unground phosphate rock. This ismade possible by using the ball mill as shown in FIG. 2 as a combinationgrinder-reactor into which the unground phosphate rock is introduced.The use of dry ground phosphate rock in this embodiment is optional.When desired, the ground phosphate rock can be introduced into theheterogeneous reactor as shown in FIG. 2. The flue gas flow in FIG. 2 isthe same as in FIG. 1. The liquid flow differences are consistent withthe objective of maximizing the acidification reaction in the ball mill.For this reason, it is desired to achieve as low a pH in the coolingtower as possible to still absorb sulfur oxides. This is accomplished bymaximizing the oxidation of the SO₂ to SO₃ rather than the increase inpH as desired in the process shown in FIG. 1 by the addition of NH₃and/or K₂ CO₃. If a pH of 2.5 to 3.5 with high sulfur oxidation degreeand rate were achieved, then the percentage of sulfuric acid could behigher thereby requiring less phosphoric acid. Therefore, the higher theoxidation, which can be achieved by use of catalyst in the gaseousstream or aeration of the liquid used in the heterogeneous reactor oroxygen absorption in a heterogeneous reactor or cooling absorber, thehigher the concentration of produced sulfuric acid which increases thesolubilization capability in the ball mill. Also, if monocalciumdihydrogen orthophosphate is added to the solution from the systemand/or monoammonium hydrogen sulfate to increase the pH, more of thehydrates of sulfur oxides can be absorbed in the cooling tower fromwhich the liqueur is brought into the ball mill to acidulate theunground phosphate rock. The less concentrated the recirculating liqueuris with respect to the sulfates and sulfites, especially the sulfates,the greater the quantities of concentrated phosphoric acid are necessaryto be blended with the acid solution produced in the cooling tower.Therefore, the control of acid activity and strength results from theblending of these acids with the objective to conduct the solubilizationreaction in the ball mill as rapidly as possible so that high percentagecompletion of the desired reaction is achieved in the ball millreactor-mixer. The more dependent upon the acid from the sulfur oxidesthe process becomes, the more important is the grinding action of theball mill so as to grind off primarily the calcium sulfate and sulfitecoatings tending to reduce the reaction rate as well as degree. Thesolid-liquid slurry from the ball mill could go through a filter, thesolids from which could go directly to a slat den or traveling beltconveyor whose rotary cutter or some means of pulverization manufacturesRun-Of-Pile (ROP) analyzing between single and triple super phosphate.The feed to the den could also come from the bottoms of theheterogeneous reactor. The solid-liquid slurry from the ball mill isshown passing into a separating system, denoted as a filter in FIG. 2,where the high solids slurry is passed to a pre-neutralizer, granulator,drier and processing to produce solid fertilizer product. Thesolid-liquid slurry from the ball mill could go to emergency storage, asshown by the dotted line on FIG. 2, should a breakdown occur. Thefiltered solution can be recycled to the heterogeneous reactor as shownin FIG. 2.

Since the sulfur oxides not absorbed in the cooling tower remain in thesaturated gas relative to moisture and the mixture flows into theheterogeneous reactor, the calcium solution would strip these sulfuroxides from the gas stream forming a slurry which would flow to asettling tank II. The decant from the settling tank can be recirculatedto the heterogeneous reactor through an additional aerator, as shown inFIG. 2, to increase the efficiency of sulfur oxide stripping. It mayalso be desirable to pass the decant through a clarifier to furtherremove solids to prevent plugging of nozzles in the heterogeneousreactor.

Another option is for the bottoms from the heterogeneous reactor to passthrough a filter which would polish the liqueur to keep from pluggingthe nozzles in the heterogeneous reactor and the solids could then go tothe drier or conveyor or granulator. If the conventional granulationplant is built in conjunction with the absorption and rock reactionsystem then the slurry from the clarifier, settling and drag tanks couldall flow to the pre-neutralizer or granulator or both, dependent uponthe concentration of solids. Also, the use of the conventionalgranulation plant allows anhydrous ammonia to be added to the mixingtank shown in FIG. 2, the slurry from which flows to settling tank III.The decant from settling tank III is passed through the clarifier orsome device for removing solids to prevent plugging of nozzles in theheterogeneous reactor. This allows for additional pH control in theheterogeneous reactor. The bottoms from settling tank III join the othersolid-liquid slurry streams for introduction to the pre-neutralizer orfor combination with the clarifier liquid stream from valve V₁₆. Theslurries going either to the pre-neutralizer or granulator or both, areprocessed to the finished solid granulator fertilizer product in thesame manner as described above with respect to FIG. 1.

The liquid fertilizer product base is taken from the recycle stream tothe heterogeneous reactor nozzles flowing from the clarifier by means ofvalve V₁₂. This liquid is conveyed to an agitated tank (not shown) towhich white potash, non-coated ammonium nitrate and other conventionalliquid fertilizer ingredients are added to formulate a desired grade.

The liquid from the clarifier may also be passed through valve V₁₆joining the solid slurry stream below valve V₁₅. The resultant mixtureconveyed to valve V₁₇ and can be used as a suspension fertilizer basesimilarly to that described with respect to FIG. 1. Therefore, it isseen that the process of this invention is a process for the productionof fertilizer comprising reacting sulfur oxide containing gas and waterin a multiple state reactor to form sulfur containing acid, reacting thesulfur containing acid with a basic ion selected from the groupconsisting of calcium, ammonium and potassium ions, the calcium beingderived from a salt selected from the group consisting of phosphatic andnitrogeneous salts to form a fertilizer selected from the groupconsisting of phosphatic, nitrogeneous and potassium fertilizers andcombinations thereof. Particularly suitable is provision of calcium ionderived from phosphate rock.

The flow of matter of gaseous, solid and liquid states through theheterogeneous reactor, cooler-absorber and other flow contact devicesinvolved in the process and apparatus of this invention may becountercurrent or cocurrent in any combination. That is, the gas streammay be cocurrent to both liquid and solid, cocurrent to one andcountercurrent to the other or countercurrent to both liquid and solidmaterials process flows.

When untreated phosphate rock is used in the process of our invention,it has been found that the sulfur containing acid must be predominantly(greater than 50%) sulfuric acid to obtain high conversion of the rockand removal of sulfur dioxide from the stack effluent. In order toobtain predominantly sulfuric acid, it is necessary to oxidize thesulfur oxides in the stack effluent. This problem becomes greater withhigher amounts of sulfur in the burned fuel. Under such conditions, wehave found it preferable to solubilize phosphate rock using a solutionof pH of about 2.5 to 4.0 and increasing the pH of the solution to a pHof about 5 to 8, preferably 5 to 7, to facilitate removal of sulfuroxides from the gas stream.

We have found that when pretreated, defluorinated phosphate rock is usedin the process of this invention, the sulfur containing acid may besulfurous acid while still obtaining high conversion of the rock andremoval of sulfur dioxide from the stack effluent of high sulfurcontaining fuels. Such pretreated, defluorinated phosphate rock has beenfound to react to a good degree with sulfur containing acid which ispredominately sulfurous acid, thus alleviating the necessity foroxidation of sulfur dioxide to obtain satisfactory solubilization of thephosphate rock. Under such conditions, we have found it preferable tosolubilize phosphate rock using a solution of pH of about 3 to 5,preferably about 3.5 to 4.5, and increasing the pH of the solution to apH of about 5 to 7, preferably 6 to 7, to facilitate removal of sulfuroxide from the gas stream.

By pretreated, defluorinated phosphate rock, we mean predominantly crudetricalcium phosphate produced by thermal defluorination of fluorapatite.Prior processes for producing defluorinated phosphate rock are reviewedin "Phosphorus and Its Compounds," Volume II, edited by John R.VanWazer, Interscience Publishers, Inc., New York, N.Y., 1961, pages1090-1092. Another suitable form of pretreated phosphate rock whichbecomes especially suitable when the principal energy source iselectrical is "calmeta" or calcium metaphosphate produced bysimultaneous oxidation of elemental phosphorous and reaction of theresulting P₂ O₅ with phosphate rock as more fully described inPhosphorus and Its Compounds, supra, pages 1095-1097. The pretreated,defluorinated phosphate rock useful in the process of this inventiongenerally contains less than about 0.2% F and a minimum of about 18% P(41.3% P₂ O₅). Untreated phosphate rock fluorapatite contains about 3.5%F. Defluorinated phosphate rock is presently available from commercialsources.

The pretreated defluorinated phosphate rock may be used permittingoperation of the process as described above at higher pH levels thanpractical with untreated rock. The pH is an important factor in bothsulfur dioxide removal and phosphate rock conversion. The pretreated,defluorinated phosphate rock has been found to remove in excess of 90%of the sulfur oxides, predominately SO₂, at the cooling tower stageforming predominately sulfurous acid. A range of pH of about 3 to 5 issuitable in the cooling tower and tank 1, pH of about 3.5 to 4.5 beingpreferred. Pretreated, defluorinated phosphate rock conversion to anammoniable fertilizer by solubilization with sulfurous acid is greaterthan 90 percent. It has also been found advantageous to pretreat thedefluorinated phosphate rock by soaking in water. Pretreating by soakingfor a few days has been found to significantly increase the rocksolubilizing. Longer times in the order of about 30 to 120 days aresuitable. The extremely high removal of sulfur dioxide from the stackgas from combustion of a high sulfur containing fuel is enhanced by arange of pH in the heterogeneous reactor of about 5 to 8, preferablyabout 5.5 to 6.5.

FIG. 3 shows one embodiment of an apparatus according to this inventionwherein a portion of the product is recycled to tower 2 as the liquidspray. The embodiment shown in FIG. 3 has been found especially usefulwhen using defluorinated phosphate rock in which the sulfur oxide may beremoved from the gas stream at a high efficiency without oxidation andthe formed sulfur containing acid, primarily sulfurous acid, has beenfound to satisfactorily solubilize the pretreated phosphate rock. Asshown in FIG. 3, sulfur containing coal is provided by feeding means 50to boiler 51 wherein it is burned and the stack gases leave the boilervia stack 62. In horizontal stack 62 is located spray 90 for cooling thehot stack gas prior to entry into tower 1. We have found that it isadvantageous to cool the stack gas to about 130° to 160° F. prior toentry into tower 1. Tower 1, shown as 52 is a heterogeneous reactorproviding enhanced liquid-gas contact for good reaction, such as theconfiguration shown as the cooling tower in FIGS. 1 and 2. The sprays intower 1 are provided by nozzles 91, 92, and 93, as well as spray 90 inthe inlet stack 62, all provided with liquid solution from tower 2effluent tank, passing through conduit 79, pump P-10 and the amount ofliquid to the sprays and tower 1 liquid effluent tank 70 beingcontrolled by valve V-32. The gas passes out of tower 1 by conduit 53into the top of tower 2. Tower 2 is any suitable heterogeneous reactordevice for the enhancement of liquid-gaseous reactions. A particularlywell suited configuration for the heterogeneous reactor of tower 2 isthe heterogeneous reactor as shown in and described in more detail withrespect to the heterogeneous reactor of FIG. 1. The gas stream passesdownward in tower 2 and is contacted by liquid from sprays 94 and 95.The gas exits from tower 2 via stack 55 having desired analyzing meanssuch as infrared analyzer 56 for SO₂ measurement and dust monitor 58.Mist suppresser 57 may be installed as necessary to prevent excessivewater vapor in the stack effluent. Blower 59 may be used to cause thegas to pass through stack 60 to the atmosphere and to provide necessaryboiler draft.

Tower 1 liquid effluent tank 70 receives liquid from conduit 81 at thebottom of tower 1 and liquid from tower 2 liquid effluent tank 77 viaconduit 78 and controlled by valve V-31. The liquid from tower effluenttank 70 passes to reactor tank 72 by conduit 71 and pump P-11, ifnecessary. The reactants for fertilizer production, except for theproduced sulfur containing acids, are added to the system by addition toreactor tank 72 and are shown as water, ammonia and phosphate rock.Solubilization of the phosphate rock takes place in reactor tank 72 andproduct from reactor tank 72 is passed through conduit 73 controlled byvalve V-30 into settling tank 74 from which liquid base fertilizerproduct is withdrawn by conduit 75.

Liquid product from tank 74 also provides the liquid spray for tower 2through nozzles 94 and 95. The liquid leaves tower 2 through conduit 82and passes into tower 2 liquid effluent tank 77. Liquid from tower 2effluent tank is passed to the sprays of tower 1 and directly to tower 1liquid effluent tank.

A pilot plant containing equipment as shown in FIG. 1 and FIG. 3 hasbeen successfully operated with an overall pressure drop between the gasinlet to the cooling tower and the gas outlet from the heterogeneousreactor of about 2 inches of water. This represents a large savings inenergy when compared with conventional scrubbers which operate in therange of 40 to 70 pressure drop of water.

Fertilizer has been produced according to the above described processand the residue after burning the coal has been ground and added to thefinal fertilizer product thereby eliminating disposal problems. Suchfertilizer has been tested on soy beans, brome grass and spring wheat.In each case, the plants grew considerably faster and to a significantlylarger size than control plants grown in good farm land topsoil.

The following Examples show specific operations utilizing theembodiments of our invention and are intended to be exemplary and not tolimit our invention in any way.

EXAMPLE I

An apparatus similar to FIG. 1 without the pre-neutralizer and equipmentfor treating solid fertilizer, was used to produce liquid fertilizerbase from fresh untreated phosphate rock. Coal containing about 3.37percent sulfur was burned in a boiler and the stack gases from theboiler having CO₂ content of 8.8 percent and excess air in the amount of89.1 percent to facilitate what little oxidation could be achieved werepassed without fly ash separation into the bottom of the cooling tower.The gas flow rate was 282 CFM at 60° F. Untreated phosphate rock havinga particle size wherein 82 percent passed a 230 mesh U.S. StandardSeries Screen and essentially 100 percent passed a 20 mesh screen wasadded to Settling Tank I at the rate of 81 pounds in 51/2 hours. Theliquid in Settling Tank I was mixed and circulated to sprays in thecooling tower at the rate of 19.3 gal/1000 CF gas primarily to keep thenozzles from plugging under the condition of operating at one-fifth ofdesigned flow rate. The pH of the liquid fed into the cooling tower was4.2 and the pH of the liquid passing from the cooling tower to SettlingTank I was 3.5. The gas was removed from the top of the cooling towerand introduced to the top of the heterogeneous reactor.

Settling Tank II, as shown in FIG. 1, was split into a catch tank forliquid from the heterogeneous reactor which feeds an ammoniating reactorfor treatment of the solution for recirculation to the heterogeneousreactor.

Water and ammonia were added to the ammoniating reactor and the liquidfrom the reactor mixed and circulated to sprays in the heterogeneousreactor at the rate of 22 gal/1000 CF gas, again to prevent pluggingunder operating conditions of reduced flow demand. The pH of the liquidfed into the heterogeneous reactor was 6.3 and the pH of the liquidpassing from the heterogeneous reactor to the catch tank was 5.9.

The overall SO₂ removal from the stack gas was 84 percent.

Liquid fertilizer product was withdrawn from the collection tanks. Thefertilizer components were nitrogen from the ammonia and phosphate fromthe rock.

EXAMPLE II

The apparatus shown in FIG. 1 modified by substituting in series a catchtank, a plug flow reactor, a back mix reactor and a settling tank fromwhich the liquid is recycled to the spray in the tower, for eachSettling Tank I and Settling Tank II shown in FIG. 1. Coal containing3.4 wt. percent sulfur was burned in a boiler and the stack gases fromthe boiler were passed to the bottom of the cooling tower.

Pretreated defluorinated phosphate rock having less than 0.18 percent Fwas hydrolized in tap water for eleven days prior to being added to theprocess. The hydrolized pretreated defluorinated phosphate rock, 100percent passing No. 8 mesh screen, was added to the plug flow reactortanks. It was shown during these runs the pretreated defluorinatedphosphate rock was sufficiently reactive with the acids formed from theexhaust gases that both coarse (100%-8 mesh, 2%-30 mesh) and fine(100%-8 mesh, 75%-200 mesh) rock particles achieved desired conversionsand SO₂ removal efficiencies. The liquid in the settling tanks was mixedand recycled to each of the towers at a concentration of reactingcalcium phosphates sufficient to absorb the sulfur oxides from thecombustion gases. Operation of the process with the pH of the liquidsolution outlet from the heterogeneous reactor at 6.35 resulted in 90.1percent SO₂ removal and a gas SO₂ outlet concentration of 142 ppm whileoperation at a pH of 5.70 resulted in 85.6 percent SO₂ removal and a gasoutlet SO₂ concentration of 207 ppm.

When ammonia was added to the process to increase the pH of the liquidsolution at the outlet of tower 2, or the heterogeneous reactor, to a pHof between 6.4 to 6.9, SO₂ removal was increased to an amount in excessof 92 percent corresponding to an outlet SO₂ concentration of about 110ppm.

In each of the above cases, water was added to the reactor tanks tocompensate for the water evaporated from the towers.

EXAMPLE III

An apparatus as shown in FIG. 3 and previously described with respect toFIG. 3, was used to produce liquid fertilizer from pretreateddefluorinated phosphate rock. Coal containing about 6.2 wt. percentsulfur on a dry basis was fed by an underfeed stoker and burned in adouble pass fire tube boiler with excess air and the stack gases passedfrom the boiler into the bottom of tower 1. As little as 20% excess airwas used in other comparable runs. The temperature of the stack gasbefore it entered tower 1 was about 150° F. due to the liquid spray inthe conduit between the boiler and tower 1. One hundred Thirty (130)pounds of defluorinated phosphate rock, containing less than about 0.18%F, was hydrolized in 60 cubic feet of tap water for two weeks. Water andpretreated defluorinated phosphate rock only were added to the reactortank to obtain pH's indicated in the fluid flow and to maintain desiredchemical conditions for removal of sulfur oxides and fertilizerproduction. It was found that with the recycle liquid pH at 6.90 at thesprays in tower 2 and a pH of 6.50 in the liquid passing from tower 2 totower 2 liquid effluent tank, sulfur dioxide removal from the stack gasof 92.5% was obtained.

It was found that when a small amount of ammonia was added to thereactor tank, in addition to water and defluorinated phosphate rock asdescribed above, removal efficiencies of sulfur oxides increased asshown in Table I and conversion of the phosphate rock and ammoniareached 90% based upon reaction products of calcium sulfite, calciumsulfate and ammonium phosphates. The sulfur dioxide removal at 15 minuteintervals is tabulated in Table I. The stack gas left tower 2 at atemperature of 110° F. and at 460 FPM.

                  TABLE I                                                         ______________________________________                                                       Sulfur                                                         Tower 2                       Percent                                         pH In     pH Out     Outlet PPM   Removal                                     ______________________________________                                        6.60      6.0        125          94.73                                       6.55      6.0        123          94.81                                       6.50      6.0        111          95.32                                       6.50      6.0        109          95.40                                       6.55      6.05        96          95.95                                       6.50      6.0        103          95.7                                        6.5       6.0        130          94.5                                        6.4       6.0        111          95.3                                        6.4       5.0        116          95.1                                        6.43      5.9        136          94.3                                        6.45      5.95        96          96                                          6.45      6.0         90          96.2                                        6.45      6.0         83          96.5                                        6.45      6.0         79          96.7                                        6.45      6.0        103          95.7                                        6.43      6.0         87          96.3                                        6.46      6.0         88          96.3                                        6.42      6.0         85          96.4                                        6.42      6.0        101          95.7                                        6.35      5.78        95          96.0                                        6.3       5.88       119          95.0                                        ______________________________________                                    

The United States of America Federal Environmental Protection Agencyallowable amount of SO₂ in the stack effluent was 177 ppm whichcorresponds to recovery of 92.5% of total sulfur. It was thus seen thatE.P.A. standards were exceeded with the burning of 6.2% sulfurcontaining coal.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

We claim:
 1. A process for the production of fertilizer comprisingreacting sulfur oxide containing gas and water at a pH of 3 to 8 in amultiple state reactor to form sulfur containing acid, maintaining saidpH of solution of said sulfur containing acid at 3 to 8 with a basic ionselected from the group consisting of calcium, ammonium and potassiumions, said calcium being derived from a salt selected from the groupconsisting of phosphatic and nitrogeneous salts, to form a fertilizerselected from the group consisting of phosphatic, nitrogeneous andpotassium fertilizers and combinations thereof, said basic ion beingrendered water soluble by reaction with said sulfur containing acid at apH of 2.5 to 5 and said solution increased to a pH of from 3 to 8 so asto facilitate the reaction of the sulfur oxide.
 2. The process of claim1 wherein said sulfur containing acid is predominately sulfuric acid,the calcium ion is solubilized at a pH of 2.5 to 4.0 and the pH of thesolution increased to a pH of 5 to 8 to facilitate the reaction of thesulfur oxide.
 3. The process of claim 2 wherein the calcium ion issolubilized at a pH of 2.5 to 3.5 and the pH of the solution isincreased to a pH of 5 to 7 to facilitate the reaction of the sulfuroxide.
 4. The process of claim 2 wherein phosphate rock is pretreated bysoaking in water.
 5. The process of claim 2 wherein the sulfur oxidecontaining gas is obtained from the combustion of fossil fuel.
 6. Theprocess of claim 2 wherein the sulfur oxide containing gas is obtainedfrom sulfur producing chemical reactions.
 7. The process of claim 2wherein calcium is derived from phosphate rock having a B.P.L. of about55 to 65.